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PWL#108 - Grinding Safety, Purging Innovations, Composite Solder, Low Alloy Steel Heat Treatment
August 01, 2012
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PWL#108
Practical Welding Letter No. 108
August 2012

PWL#108 - Grinding Safety, Exacting Purity Requirements drive Purging Innovations, Innovative Composite Solder Filler Metal, Heat Treatment of Low Alloy Steel, Constitution Diagrams, Orbital Welding (R), Braze Welding (R) and much more...


August 2012 - Practical Welding Letter - Issue No.108



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TABLE of CONTENTS
1 - Introduction

2 - Article - Grinding Safety

3 - How to do it well: Exacting Purity Requirements drive Purging Innovations

4 - Innovative Composite Solder Filler Metal

5 - Online Press: recent Welding related Articles

6 - Terms and Definitions Reminder

7 - Article: Heat Treatment of Low Alloy Steel

8 - Site Updating: Orbital Welding (R), Braze Welding (R)

9 - Short Items

10 - Explorations: beyond the Welder

11 - Contributions: Constitution Diagrams

12 - Testimonials

13 - Correspondence: a few Comments

14 - Bulletin Board

(Sponsored Links)


1 - Introduction

This issue, coming during Vacation period, will have even less chance of being read with some attention. Have a nice time! Enjoy life and Family. You can always come back and read it whenever you feel in the right mood.

Anyhow we start by reporting an article on Grinding Safety, possibly a neglected subject. Given the dangers of accidents, attention should be devoted to avoid their occurrence. It is not difficult but it needs some good old discipline.

Delicate pharmaceutical and food industries drive stricter requirements on cleanliness and purity, affecting welding of stainless steels. New devices become available to help meeting requests.

A new composite solder is introduced for somewhat specific an application. The reasoning affecting such development may be applicable also to different cases.

A comprehensive article is possibly required as a reminder of basic facts affecting Heat Treatment of Low Alloy Steels. Also brushing up the concept of Hardenability may be useful if one faces disappointing results.
It may be useful to refresh basic facts.

The article in Section 11 provides essential information on the microstructures to be expected when welding stainless steels. Whoever needs more detailed knowledge is urged to procure the booklet indicated at the end of that article: it is a Handbook monograph of 31 pages likely to shed light on uncertain issues.

The pages of this month are new editions of old pages, hopefully more informative and useful than the original ones. Orbital Welding (R) includes now recent innovations, and Braze Welding (R) was updated as necessary. Comments and feedback are welcome.

Please visit the Site Map and the Index Page to find what you need, and use the Contact Us form to send us your correspondence.


2 - Article - Grinding Safety

Many Safety issues are probably dismissed as nuisances, as they take time and attention away from production, the main concern of any job. Nevertheless, Safety requires special commitment on the part of Management and constant surveillance from supervisors and workers.

It is however unforgivable to conduct a sloppy operation without due care to the many dangers looming over industrial activities. The cost in pains and anguish born by victims of accidents should be kept always in mind by all involved.

These considerations came to mind while reading a short note on Grinding Safety published on page 32 of the July 2012 issue of the Welding Journal. It is part of a larger article dealing with abrasives used for finishing stainless steel welds for food processing equipment.

Those working with hand held grinding equipment are urged to obtain the referred note and translate it into mandatory directives to be enforced on all those using such implements.

Suitable personal safety equipment should be available. Workers should not be allowed to perform grinding without wearing properly the different items of a complete set. They should be instructed on the dangers specific to grinding operations and on the precautionary protections to be adopted.

In particular grinding wheels for right-angle grinders should be checked frequently and used only to their maximum speed, because they may shatter in pieces flying around at high speed. Regular maintenance of equipment and of used wheels is a must that has to be implemented by strict discipline.

Interested readers are urged to seek the original publication. Those who may have first hand information on related stories are invited to share them with our public by contributing a note to this newsletter.


3 - How to do it well: Exacting Purity Requirements drive Purging Innovations

Traditional practice required back side purging for certain root welds. This was achieved in the past by simple means assuring protective argon flow arranged locally.

More recently however, bioprocessing industries, food and pharmaceutical production and semiconductor manufacturing operations, developed increased purity requirements for equipment involved, especially concerning pipe work.

These requirements were formalized in the latest version of:

ANSI/ASME B31.3-2010
Process Piping
American Society of Mechanical Engineers / 31-Mar-2011 / 400 pages
Click to Order.

An article introducing the new requirements was published at page 36 of the July 2012 issue of the Welding Journal. The following sentence is enlightening.

"In the food processing industries, statutory legislation and a plethora of litigation suits have forced plant manufacturers to introduce quality control levels previously considered unnecessary. Contamination introduced during fabrication is now unacceptable."

To satisfy these requirements, special purging devices were developed, essentially delimiting a certain volume in the pipe where welding has to be performed. This is achieved by using specially made expanding plugs, through which back face flowing shielding gas is provided.

Heat resisting materials are employed for those cases where preheat or post weld heat treatment operations are required. With such implements it becomes possible to meet the new specification demands.

Interested readers are urged to seek the original article indicated above. Whoever has experience with this kind of work is invited to contribute with a short note.


4 - Innovative Composite Solder Filler Metal

The voted best soldering paper presented this spring at the 5th International Brazing and Soldering Conference (IBSC) held in Las Vegas, Nev. is now reprinted at page 50 in the July issue of the Welding Journal.

The high-temperature (280°C or higher) solder alloys cover a special need in die-attach, power semiconductor and optical device packaging, in that it allows further mounting of details and devices soldered at a lower temperature (200° to 250°C).

Due to the current lack of suitable lead-free (Pb = poisonous metal) solders covering these needs, for these specific purposes, there is now a temporary moratorium to the universal ban.

The quest for an acceptable substitute is driven by this urgent need. Several candidate compositions were already screened out because of high cost or of intrinsic drawbacks.

The reported development work is based on the principles of transient liquid phase (TLP) bonding. See PWL#023, section 6.7.

The composite lead-free solder is made of a ductile Ag (silver) metal core sandwiched between two low-melting SnAg based solder coating layers. Upon melting, the solder coating forms Ag-Sn inter metallic compounds (IMC) with the adjacent metals (core and substrate), with much higher remelt temperature.

Despite the hardness of the IMCs, the joint shows sufficient ductility due to the presence of the soft core silver metal, maintaining resistance to brittle fracture. The recommended soldering procedure involves the use of a suitable flux and the application of moderate pressure to reduce the extent of voids in the joint.

Test results and joint examinations show encouraging reliability, while further refinement of materials and procedures are required to optimize the joining process and composite preform design.

Interested readers are urged to seek the original article referred to above.


5 - Online Press: recent Welding related Articles and Video

Welding Pelton Runners
Pelton.

Tips on Plasma Cutting - Video
http://media.thefabricator.com/pwtv/webisode7.cfm

Ready on the day - Underwater Inspection - TWI Video
http://www.youtube.com/watch?v=yd5qmu4dE38&feature=plcp

Connect - Issue 178 - May/June 2012
http://www.twi.co.uk/news-events/publications/connect/may-june/

Laser Welding of large Format Metal-Foam Sandwich Materials
LZH.


6 - Terms and Definitions Reminder

Electrode Skid is sliding of resistance welding electrodes on the surface to be welded while performing spot, seam or projecion welds.

Flashover is unwanted arcing or sparking within the electron beam gun, in electron beam welding.

Heating Gate is an opening in a thermite mold, through which a flame is directed for preheating.

Intermediate Weld Pass is a single progression of weld deposition performed between root and cover passes.

Joint Brazing Procedure specifies materials, joint design, method and practices to be used for performing a particular joint brazing.

Longitudinal Sequence is the exact order in which successive portions of a continuous weld pass must be deposited, in order to control heat input and distortion.

Magnetically Impelled Arc Welding is a process in which an arc is created between the abutting ends of two tubes. The arc is driven around the joint by a magnetic field. Upon sufficient heating of the tube ends, welding is performed by upsetting.

Powder Composite is the combination of two or more materials to form a single particle, either by chemical coating or by mechanical agglomeration.


7 - Article: Heat Treatment of Low Alloy Steel

I was amazed to find out recently in a renowned professional forum some questions regarding the hardness that can be obtained by heat treating the quite common low alloy steel designated AISI 4140. The questions and the discussion revealed a measure of confusion about basic facts.

Following that hint I concluded that an overview of the important characteristics both of the materials and of the heat treating process might be useful to the readers of this publication.

Before attempting to heat treat any of the low alloy steels one should remember the concept of hardenability. For a description of this property see the relevant Hardenability Page.

One can quantify the property by a standard test (Jominy). Or one may describe qualitatively the relative ease or difficulty to develop in the material, by a suitable heat treating process, the required microstructure exhibiting the sought mechanical properties.

Let us examine first plain carbon steel. To develop any measurable hardness upon quenching from the austenite temperature (about 820-840°C) one needs:

  • medium carbon content (at least 0.40%C)
  • the physical possibility to extract heat as quickly as possible.

The first condition depends on chemical composition and can be ascertained beforehand. The second one depends on the geometrical features of the object one has to heat treat.

It is obvious that a thick block cannot be quenched rapidly enough in cold water to develop significant hardness, because the heat flow speed from the interior to the surface is a fact that cannot be increased by any means, except by reducing surface temperature.

The above requirements originate from the fact that the transformation microstructure called martensite, displaying the required hardness and strength, can only be obtained by quick heat removal, except for specially alloyed steels. With slower cooling, other microstructures develop, less hard and strong.

The martensitic transformation is diffusionless and it is progressing only when the material is cooling. Upon cooling, more and more tiny volumes of the original structure (generally austenite) suddenly transform, but the process stops if cooling is suspended. The amount of martensite is dependent on temperature and independent on the time at temperature.

Alloying steel with other metals by altering the composition at the steelmaking stage, may have interesting effects on the microstructures that form during thermal cycling.

Going back to low alloy steel one will remark that the main difference between AISI 4140 and AISI 4340 is hardenability, deriving from composition difference. Both materials have about 0.40%C (medium carbon) and can get about the same hardness level.

AISI 4140: 0.40%C 0.95%Cr 0.20%Mo
AISI 4340: 0.40%C 0.80%Cr 0.25%Mo 1.82%Ni

But while 4140 is fully hardenable only up to about 0.5" size, 4340 can be hardened up to about 3". This means that although the cooling rate is slow in the thick bar, the alloying additions permit nevertheless the sluggish transformations to develop the martensitic microstructure.

Steelmaking, except for special materials, is normally performed in air. This means that the carbon content of the surface cannot be taken for granted. The machining allowance expresses the amount of material that must be removed to reach the nominal carbon content.

If surface hardness is important on should remove by machining the decarburized layer. Normal air heat treatment furnaces do not protect from decarburization. If hardness of machined surfaces is important one should either machine after heat treatment, or not use air furnaces but those with protective atmospheres or vacuum.


8 - Site Updating: Orbital Welding (R), Braze Welding (R)

The pages of this month are new editions of older pages. The revision involved the whole text and the reported references. It is believed that the updates are correct and more useful. Readers who wish to comment on what they read are invited to let us know their opinions by using the the Contact Us form to send us their correspondence.

The pages above are found at:
Orbital Welding
and
Braze Welding

Please visit the Site Map and the Index Page to find what you need, and use the Contact Us form to send us your correspondence.


9 - Short Items

9.1 - Reduction of Area (RA) is the difference between the original cross-sectional area of a tensile specimen and the smallest area after fracture, referred to the original area and expressed in percent.

9.2 - Subcritical Annealing is a thermal annealing treatment in which a steel is heated to a temperature below the A1 transformation temperature, then cooled slowly to room temperature.

9.3 - Trimming describes the operation of removing unwanted material, generally along the partition plane of a mold. In forging, removing any parting-line flash or excess material. In drawing, shearing the excess irregular edge of the drawn part. In casting, the removal of accessory needed metal volumes, known as gates, risers, and fins.

9.4 - Uniaxial Stress is a state of stress in which two of the three principal stresses are zero.

9.5 - Vacuum Deposition is obtained by metal evaporation techniques of a metal film onto a substrate, in a vacuum.

9.6 - Wrought Iron is an iron alloy with very low carbon content which has fibrous inclusions, known as slag (iron silicate) fibers in a ferrite matrix. Wrought iron is tough, malleable, ductile and easily welded but it is no longer produced on a commercial scale, being substituted by mild steel.


10 - Explorations: beyond the Welder

Primordial Pinwheel
SA1.

Mars Science Laboratory (Curiosity Rover) Mission Animation - NASA Video
NASA.

Does Thinking Really Hard Burn More Calories?
SA2.

The Higgs boson and the future of science
SA3.

New Biomarkers Honed to Help Search for Life on Earthlike Exoplanets (3 pages)
SA4.


11 - Contributions: Constitution Diagrams

Our page, a general overview on Stainless Steel Welding explains, among other things, the subdivision in types, suitable for most diverse applications.

It is however quite a complex subject that includes considerations of different aspects. The present note intends to shed some light on a number of issues concerning specifically the large class of Austenitic Stainless Steels.

These materials are chosen for their considerable resistance to corrosion in a wide range of service conditions. When fabrication calls for the use of welding, the selection of suitable filler metals must meet two separate requirements, besides that of sensitization, dealt with in the above page.

In the first place, the corrosion resistance of the filler should be comparable to that of the base metal. Then its composition should include a certain amount of ferrite, a common remedy to oppose the tendency of hot cracking during welding.

To help in the selection of suitable materials, one can usefully resort to constitution diagrams. These started with the well known Schaeffler Diagram (1949) that provides estimates of stainless weld metal microstructures, based on composition.

The horizontal coordinate expresses the chromium equivalent, a single number calculated from the percentages of chromium and other ferrite promoters by an empirical formula.

The vertical coordinate represents the nickel equivalent, to describe the influence of austenite promoting elements. By plotting the calculated equivalent coordinates of the composition of a weld bead, one gets a point placed in a zone expressing a graphic estimate of the relevant microstructures.

The area of the diagram is divided by boundaries delimiting zones designated by the names of the main microstructures present, Austenite, Martensite and Ferrite or mixtures thereof.

Experience has shown that for the common stainless of the 300 series, the estimates of the original diagram are still quite accurate. The same can be said of martensite predictions in lean stainless compositions.

Further research permitted to refine successively constitution diagrams, so that the current one is the (Welding Research Council) WRC-1992, useful for stainless materials other than the 300 series.

The Ferrite Number, designated FN, has become accepted as a useful indicator of volume ferrite percent. Several instruments provide this value based on magnetic properties of the sample. Users should be aware of the difference in results obtainable from different devices and on the need to calibrate them periodically.

ANSI/AWS A4.2M:2006 (ISO 8249:2000 Mod.)
Standard Procedures for Calibrating Magnetic Instruments to Measure Delta Ferrite Content of Austenitic and Duplex Ferritic-Austenitic Stainless Steel Weld Metal
American Welding Society / 10-Jul-2006 / 58 pages
Click to Order.

For practical purposes it is generally accepted that, for austenitic weld deposits, FN about 4 or 5 (minimum) should prevent hot cracking. However many exceptions exist, depending on influence of possible post weld heat treatment, on actual impurity level, on welding practices (keeping a short arc) and cooling rates.

By plotting separately on WRC-1992 the calculated equivalent coordinates obtained from the chemical analysis of base metal and of filler material, one understands that the weld metal obtained, being an alloy of the two, will be found on the line connecting the two points.

Where exactly, it is likely to be dependent on the amount of dilution. For 30% dilution the weld metal microstructure is likely to be found on that line at 30% (of the segment) from the base metal point.

This exercise helps in forecasting the outcome of welding of any base-metal/filler combination and in obtaining advance notice of possible problems.

For further information on the subjects only hinted at in these notes, interested readers are strongly advised to procure the Chapter on
Welding of Stainless Steel from Volume 6 of ASM Handbook.

To get it, type in your browser:
http://www.asminternational.org/portal/site/www/AsmStore/
Type the title above in the search box and Go
Click on the exact Title as above:
You will be offered the said chapter (31 pages) for direct download.
Price: $ 30.00 - Member Price: $ 24.00


12 - Testimonials

Date: Mon, 2 Jul 2012 05:27:43 +0000
Name: J. Carlos Glez
E-mail: removed for security
To: welding-advisers.com
Subject: RE: PWL#107- Progress in Orbital Welding, Microfissures in Stainless Steel, [...]

Ok thank you, is very good information.


Name: Dale Loiselle
Date: 14 Jul 2012, 08:45:47 AM
Certified Welding Inspector, CWI
AWS Board Member - District 12 Local Section 14
To: welding-advisers.com
Subject: Dale, PRACTICAL WELDING LETTER Unsubscribed
Elia,

I'm not unsubscribing because you did *not* meet my needs. I do like the email magazine but lately the email has caused my email program (Lotus Notes) to shut down when I click on PRACTICAL WELDING LETTER to read it. I have to reboot every time this happens.
I don't know why this happens and our tech people here don't have an answer for me for why this happens.

Believe me I feel a loss to not be able to read PRACTICAL WELDING LETTER in my in box.

I may attempt a subscription in the future.

Regards,

Dale Loiselle


13 - Correspondence: a few Comments

13.1 - While looking for interesting subjects to be dealt with in these pages, it has dawned on me that I know practically nothing on the main occupations and interests of most of the readers. Therefore I would like to conduct a wide questionnaire, sometime later on.

I would like to hear from you any suggestions you may like to propose to improve the value of this publication.

13.2 - Readers keep sending me back my newsletter as a means of finding my address. I am trying to call more attention to the print in red characters, because my warning seems to be invisible to hurried readers.

Any suggestion on how to convince readers that this is not needed?
Use Contact Us instead!.

13.3 - Next Mid Month Bulletin and the following ones will not be sent to your mailbox. You will receive notice of their publication in the RSS Blog, if you subscribed to it, and you wiil be able to download them.

Or you will find them in the Welding Resources as of their updating.


14 - Bulletin Board

14.1 - 15th Annual Aluminum Welding Conf.
Sept. 18, 19, Seattle, Wash.
www.aws.org/conferences.

14.2 - FABTECH 2012
November 12–14 - Las Vegas, Nev. - Las Vegas Convention Center.
www.fabtechexpo.com
www.aws.org/conferences

14.3 - Fusion Line - Speciality Welds Newsletter - Issue 33 – summer 2012
Download your copy from:
http://www.specialwelds.com/news/newsletter.asp

14.4 - What's stopping YOU from trying SBI!?
Watch the Video at
Try SBI!.

14.5 - Visit Now the BlockBuilder 2 Page

Get Ready for a New Start!


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